1. Field of the Invention
This invention relates to the preparation of inorganic pigments. More specifically, it relates to the preparation of ferriferrocyanide pigments, also known as iron blue pigments.
2. Description of the Prior Art
Ferriferrocyanide pigments are well known in the art. The term "ferriferrocyanide pigments" covers a variety of pigments with various chemical formulae. The chemical formula of the pigment will depend upon what the starting materials are and the specific reaction conditions. For a discussion of the various ferriferrocyanide pigments, see Temple C. Patton, Pigment Handbook, pages 401-407 (John Wiley & Sons, New York, 1973). This reference and the patent literature disclose that ferriferrocyanide pigments are prepared by the reaction of a solution of sodium ferrocyanide and ferrous sulfate in the presence of ammonium sulfate which forms the so-called Berlin White, Fe(NH.sub.4).sub.2 Fe(CN).sub.6. The Berlin White is digested in sulfuric acid and then oxidized with sodium bichromate or chlorate to give the ferriferrocyanide pigment. The pigment is washed, pressed, dried, and ground into a finished product. Several patents are quite specific in requiring that the reaction of the sodium ferriferrocyanide and ferrous sulfate be carried out in the presence of ammonium sulfate and do not suggest any substitutes for the ammonium sulfate. See for example U.S. Pat. Nos. 2,275,929, 2,592,169, and 3,021,191 all of which require that ammonium sulfate be used in the preparation of the ferriferrocyanide pigment described therein.
The use of ammonium sulfate creates problems with respect to processing. The process is expensive because the Berlin White intermediate which is formed must be dissolved in sulfuric acid and it is necessary to dispose of the sulfates formed. Therefore, it would be advantageous if a process could be developed which eliminated the formation of the Berlin White and the need to dissolve it in sulfuric acid.
U.S. Pat. No. 2,622,037 describes a process for preparing ferriferrocyanide pigments without the formation of Berlin White. The process comprises mixing an aqueous solution of a divalent iron salt with a water solution of a water soluble salt of hydroferrocyanic acid to form a mixture, and treating the resulting mixture with nitrous acid to form an insoluble ferriferrocyanide compound. This process is quite different from the process which will be described herein. It does not utilize an alkaline compound and it requires the use of nitrous acid.